Need more I/O? Use device aggregation to combine your audio interfaces into one logical device. (You may need to switch the master clock to avoid clicks.)
Got an extra computer audio interface lying around? If you're running Mac OS X 10.4 or later, you can aggregate the interface with other audio devices (USB, FireWire, PCI, or built-in) into one giant combination interface. I've used this trick to run two USB mics at once. I also created a quadraphonic speaker system by aggregating a Zoom H2 recorder (in USB interface mode) with my Mac's built-in audio output and making the H2 the master clock. That let me hear the H2's 4-channel recordings in glorious surround.
To aggregate audio devices, plug them all in and open Audio MIDI Setup. Select Open Aggregate Device Editor from the Audio menu. Click on “+” to create a new device, and check off the I/O you want. Windows users can explore device aggregation with the free Universal Driver from CEntrance.com, though it works only with CEntrance hardware. (For more about David Battino's work, visit batmosphere.com.)